In the production of rubber reinforced latex such as acrylonitrile-butadiene-styrene (ABS), methacrylate-butadiene-styrene (MBS), acrylonitrile-styrene-acrylate (ASA) by emulsion polymerization, it is industrially important to improve the efficiency of rubber reinforcement and increase productivity.
Increased productivity results in a competitive price, therefore, companies have invested much and tried various techniques, in particular they have tried to shorten the reaction time, to increase the rubber content in latex and/or to maximize the solid content.
Even though the methods introduced so far are all effective in increasing productivity and rubber reinforcement efficiency, they have the significant problem of decreasing polymerization conversion rate in the rubber reinforced latex production.
The decrease of polymerization conversion rate leads to a decrease of yield and at the same time can be a crucial factor for increasing the content of residual volatile matters (monomers and other additives). It is also important to minimize the contents of such residues to satisfy consumers by having improved perceived quality, discernible by the five senses, and to meet the environmental regulations and to maintain a high standard of living.
Generally in emulsion polymerization, polymerization conversion rate rapidly increases during the early induction period. But, from the point of 94% conversion, the speed of polymerization conversion rate becomes less. This phenomenon is especially conspicuous in rubber reinforced latex, in which the monomer content drops rapidly from when the polymerization conversion rate reaches 94%. It is believed to be caused by the interruption of an initiator for permeation of monomers into a rubber particle. Short reaction time might result in a decrease of the polymerization conversion rate. In the meantime, high rubber content and high solid content makes the migration of monomers into the inside of rubber particles easy, causing a decrease of the polymerization conversion rate.
Methods have been proposed to reduce residual monomers during polymerization. German Patent No. 19741188 describes a method that uses a long-chained initiator for the polymerization. WO No. 00/14123 described a method to treat α-hydroxy carbonyl compound with an antioxidant. U.S. Pat. No. 6,620,883 introduces multiple initiator treatment. WO No. 03/010214 describes a method including the additional insertion of a third monomer such as acrylate. The method to increase drying efficiency after the polymerization process or to increase the vacuum during extruding and injecting has also been suggested.
However, there is a limit to the ability of those methods to eliminate residual monomers. Furthermore, the physical properties of the resin itself could be changed according to the operations of those methods or the productivity might be decreased by a longer reaction time.
Therefore, a method is provided by the present invention for preparing a graft rubber latex that maximizes the productivity of polymerization without reducing the properties of the resin, minimizes residual monomer content and thereby increases yield and perceived quality of the product.